1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device having a narrow bezel region due to a light emitting diode (LED) backlight unit.
2. Discussion of the Related Art
A liquid crystal display (LCD) device that has been widely used for a television or a monitor because of its superiority in displaying a moving image and high contrast ratio produces images by using optical anisotropy and polarization properties of liquid crystal molecules. The LCD device includes a liquid crystal panel that is composed of facing two substrates and a liquid crystal layer between the two substrates. An alignment direction of liquid crystal molecules in the liquid crystal panel is changed by an electric field so that the liquid crystal panel can produce difference in transmittance.
Since the LCD device is a non-emissive type display device, an additional light source is required. Accordingly, a backlight unit including a light source is disposed under the liquid crystal panel. The backlight unit may be classified into a direct type and an edge type according to a position of the light source. In the direct type backlight unit, the light source is disposed under the liquid crystal panel and the light from the light source is directly supplied to the liquid crystal panel. In the edge type backlight unit, a light guide plate is disposed under the liquid crystal panel and the light source is disposed at a side of the light guide plate so that the light from the light source can be indirectly supplied to the liquid crystal panel using refraction and reflection in the light guide plate.
Here, one of a cold cathode fluorescent lamp (CCFL), an external electrode fluorescent lamp (EEFL) and a light emitting diode (LED) may be used as the light source. Specifically, the LED has been widely used as a light source for a display device because of its advantages such as small size, low power consumption and high reliability.
FIG. 1 is a perspective view showing a light emitting diode assembly of a backlight unit according to the related art, and FIG. 2 is a cross-sectional view showing a liquid crystal display device including a backlight unit according to the related art. In FIG. 1, a light emitting diode (LED) assembly 29 includes a plurality of LEDs 29a and an LED printed circuit board (PCB) 29b where the plurality of LEDs 29a are mounted. The plurality of LEDs 29a area spaced apart from each other. Leads 29a-1 are formed at both sides of each of the plurality of LEDs 29a and a circuit pattern 29b-1 is formed on the LED PCB 29b. The leads 29a-1 are electrically connected to the circuit pattern 29b-1 by soldering. A backlight unit 20 (of FIG. 2) of a liquid crystal display (LCD) device 1 (of FIG. 2) may include the LED assembly 29 as a light source.
In FIG. 2, an LCD device 1 includes a liquid crystal panel 10, a backlight unit 20, a main frame 30, a top frame 40 and a bottom frame 50. The liquid crystal panel 10 displaying an image includes first and second substrates 12 and 14 facing and spaced apart from each other and a liquid crystal layer (not shown) interposed therebetween. The liquid crystal panel 10 further includes first and second polarizing plates 19a and 19b on outer surfaces of the first and second substrates 12 and 14, respectively.
The liquid crystal panel 10 and the backlight unit 20 are integrated by the main frame 30, the top frame 40 and the bottom frame 50. The main frame 30 having a rectangular ring shape surrounds an edge portion of the liquid crystal panel 10 and the backlight unit 20. In addition, the top frame 40 covers a front edge portion of the liquid crystal panel 10 and the bottom frame 50 covers a rear surface of the backlight unit 20.
The backlight unit 20 is disposed under the liquid crystal panel 10. The backlight unit 20 includes a light emitting diode (LED) assembly 29, a reflecting plate 25, a light guide plate 23 and a plurality of optical sheets 21. The LED assembly 29 is disposed under the main frame 30 and the reflecting plate 25 having a white color or a silver color is disposed over the bottom frame 50. In addition, the light guide plate 23 is disposed over the reflecting plate 25 and the plurality of optical sheets 21 are disposed over the light guide plate 23.
The LED assembly 29 of the backlight unit 20 includes a plurality of LEDs 29a emitting a white-colored light and an LED PCB 29b where the plurality of LEDs 29a are formed. A temperature of the plurality of LEDs 29a increases according to an operating time and a brightness of the plurality of LEDs 29a is changed according to the temperature of the plurality of LEDs 29a. Specifically, when a heat generated by the plurality of LEDs 29a is not efficiently dissipated, a color of light emitted from the plurality of LEDs 29a may be changed. In addition, brightness of the light emitted from the plurality of LEDs 29a may decrease and lifetime of the LED assembly 29 may be reduced.
Accordingly, the LED PCB 29b is fabricated so that the heat can be dissipated from the plurality of LEDs 29a. Although the heat may be directly dissipated from the plurality of LEDs 29a to the external air, the heat dissipation efficiency from the plurality of LEDs 29a to the external air is smaller than the heat dissipation efficiency from the plurality of LEDs 29a through the LED PCB 29b. As a result, since most heat of the plurality of LEDs 29a on the LED PCB 29b is dissipated from the plurality of LEDs 29a through the LED PCB 29b, the heat dissipation of the LED assembly 29 has a limitation.
The plurality of LEDs 29a of the LED assembly 29 face an incident surface of the light guide plate 23 such that a top surface of each of the plurality of LEDs 29a is spaced apart from the incident surface by a first gap distance A1 and a top surface of the LED PCB 29b is spaced apart from the incident surface by a second gap distance A2. The second gap distance A2 corresponds to a sum of the first gap distance A1 and a width of each of the plurality of LEDs 29a. The separation between the LED assembly 29 and the light guide plate 23 by the first gap distance A1 is required for protecting the plurality of LEDs 29a by preventing contact between the incident surface and the plurality of LEDs 29a even when the light guide plate 23 expands toward the LED assembly 29 by a heat.
However, since the LED assembly 29 is separated from the light guide plate 23 by the first gap distance A1, the whole light from the plurality of LEDs 29a of the LED assembly 29 does not enter the incident surface of the light guide plate 23 such that a portion of the light is lost.
Recently, the LCD device having a wide display area and a narrow bezel region has been widely researched for a portable computer, a desktop computer and a wall-hanging television. However, since the LED assembly 29 is separated from the light guide plate 23 by the first gap distance A1, it is difficult to reduce a width W of the bezel region corresponding to a sum of the first gap distance A1, the thickness of each LED 29a and the thickness of the LED PCB 29b in the LCD device 1.